Cellulose, consisting of polymeric chains of glucose, is the most plentiful form of biomass in the world. If cost-effective methods are developed to depolymerize it, cellulosic biomass could become the basis for renewable fuels and commodity chemicals. To date, much of the research and development effort in advanced biofuels has focused on biological approaches that employ enzymes to hydrolyze cellulose to monosaccharides (Sun & Cheng, Bioresource Technol. 83:1 (2002); Dwivedi et al., Energy for Sustainable Development 13:174 (2009)). Commonly overlooked are thermal processes for producing water soluble carbohydrates that is suitable for fermentation or catalytic upgrading.
Fast pyrolysis, i.e., rapid heating of pure cellulose in the absence of oxygen, results in glycosidic bond breaking, liberating anhydrosugars in a process referred to here as carbohydrate depolymerization. Pure cellulose readily depolymerizes during pyrolysis at temperatures ranging 350 to 600° C., yielding predominately 1,6-anhydro-beta-D-glucopyranose, commonly known as levoglucosan (LG), and other anhydrosugar derivatives of glucose (Patwardhan et al., J. Anal. App. Pyrol. 86:323 (2009); Sun & Cheng, Bioresource Technol. 83:1 (2002)). Levoglucosan yields from pure cellulose can be as high as 59 wt %.
However, anhydrosugar yield from fast pyrolysis of most naturally-occurring biomass is low due to the inherent alkali and alkaline earth metal (AAEM) content contained in biomass. The presence of AAEM catalyzes saccharide ring fragmentation during pyrolysis which is in competition with glycosidic bond breaking Naturally-occurring AAEMs found in biomass catalyze the pyrolytic decomposition of cellulose to yield a preponderance of light oxygenated compounds, such as formic acid and hydroxyacetaldehyde, rather than depolymerization to simple sugar (Patwardhan et al., Bioresource Technol. 101:4646 (2009); Dwivedi et al., Energy for Sustainable Development 13:174 (2009)).
Because AAEM salts are water soluble, washing biomass with water or dilute acid has been proposed as a pretreatment of biomass to remove AAEMs and increase LG yields upon pyrolysis (Das et al., Biomass Bioenerg. 27:445 (2004); Fahmi et al., Fuel 87:1230 (2008); Brown et al., Acs. Sym. Ser. 784:123 (2001); Shafizadeh et al., J. Appl. Polym. Sci. 23:3525 (1979); Scott et al., J. Anal. App. Pyrol. 57:169 (2001); Ponder & Richards, Biomass Bioenerg. 7:1 (1994); Huber et al., Chem. Rev. 106:4044 (2006)). However, alkali metals are powerful catalysts during pyrolysis. If AAEMs are to be removed by washing, they must be substantially removed from the biomass to affect sugar yields. Such thorough washing and subsequent drying of the biomass prior to pyrolysis has been impractical or costly.
The present invention is directed to overcoming these and other deficiencies in the art.